Barium ferrite compounds (compounds of barium, iron, and oxygen) have been widely studied because of their magnetic properties. One such compound is the magneto-plumbite phase, (BaFe.sub.12 O.sub.19) which is frequently referred to as the M or M--Ba phase. The magneto-plumbite phase has hexagonal symmetry and is composed of layers of close-packed oxygen planes with iron atoms occupying several different interstitial sites and a barium atom substituting for oxygen on every fifth layer. The unit cell is composed of two formula units of BaFe.sub.12 O.sub.19. The magnetic moments of the iron atoms at the interstitial sites are parallel to the crystallographic c-axis and partially compensate each other, leading to a ferrimagnetic state below the Curie temperature of approximately 450.degree. C. At room temperature the material possesses large uniaxial crystal anisotropy and an easy magnetization axis parallel to the crystallographic c-axis. For fields applied parallel to the easy magnetization axis, coercivities larger than 6000 Oe have been reported in small single crystal particulates and nearly 5000 Oe has been achieved in thin films of the M phase.
Barium ferrite has been used as a magnetic recording medium in the form of single crystal particulates suspended in a binding material. The particulates in these media can be oriented by applying a magnetic field to the structure during the curing of the binder. The magnetic performance of barium ferrite particulate media is surprisingly close to that of a conventional thin film cobalt-alloy magnetic disk. The exceptional performance of this particulate media has been attributed to its large anisotropy field and the magnetic decoupling of the particulates. Nevertheless, in the drive toward higher density recording disks, the ability to make cobalt-based alloy media much thinner than particulate media has made it preferable for use as a recording medium.
Barium ferrite has also been investigated for use as a thin film perpendicular recording medium. Perpendicular recording media store information in the form of magnetized regions wherein the magnetization in each region is substantially perpendicular to the plane of the medium. This is in contrast to conventional longitudinal recording techniques wherein the magnetized regions have fields that lie substantially in the plane of the medium. Because the easy magnetization axis of barium ferrite is parallel to the c-axis, the prior art has been directed toward developing thin films with a high degree of c-axis perpendicular orientation. In addition to high coercivity, such films are advantageous because they are wear resistant and exhibit a high resistance to corrosion.
A favored preparation technique of perpendicular barium ferrite films is to sputter deposit barium ferrite from stoichiometric, ceramic targets in a system having two face-to-face targets onto a heated substrate located at the periphery of the plasma between the targets. This arrangement minimizes the bombardment of the thin film by energetic ions and electrons during film growth, thereby preventing changes in the film stoichiometry caused by the preferential re-sputtering of barium. Substrate temperatures during deposition range between 550.degree. and 700.degree. C. and sputtering gases are typically a few percent oxygen in argon at a total pressure of 2-8 millitorr.
While such films have desirable properties, they are of limited utility in longitudinal recording applications, which are often a preferred mode of magnetic recording. Longitudinal recording is preferred in many instances because of the relative ease with which high-performance read/write heads can be fabricated to work in conjunction with a longitudinal medium. The unsuitability of traditional barium ferrite thin films for such applications is due to their strong crystallographic and magnetic orientation perpendicular to the plane of the film. This results in an in-plane coercivity and an in-plane remanence that are far too low to be useful in high-performance recording applications.
For the foregoing reasons, there is a need for a thin film magnetic recording medium that utilizes the advantageous magnetic, structural, and chemical properties of thin film barium ferrite without the limitation of being strongly oriented perpendicular to the plane of the film.